iCloud manages your Photo Stream efficiently so you don’t run out of storage space on your iPhone, iPad, or iPod touch. If you have Photo Stream enabled on your iOS device, every single photo you take appears in a special Photo Stream album that holds your last 1000 photos. You can delete any photos you don’t want from the Photo Stream.1 To touch up a photo or keep a favorite shot permanently, simply save it to your Camera Roll. iCloud stores new photos for 30 days, so you have plenty of time to connect your iOS device to Wi-Fi and make sure you always have your most recent shots handy.

So your photos will get deleted if you reach the 1000 photos limit or the 30 days limit. It's important to note that photos stored in iCloud by Photo Stream doesn't count towards your storage limit.

Photos stored in iClouds photo stream are only stored for up to 30 days and up to 1000 photos (whichever you reach first). You should save any photos you want to keep to your photo roll or iPhoto within the 30 days allocated.

Thanks for your answer. Sorry, I m missing the point. I only have one iPad and I need to backup my photos. If I upload them to iPhoto, they will be deleted in one month, correct? So where/how do I back them up?
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p.a.Aug 16 '12 at 8:11

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You don't need to save them to your Camera Roll as photos in Photo Stream are stored locally on your devices when downloaded. So while they are removed from Apple's servers, the items currently available on your device will not disappear or be removed.
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cksumAug 16 '12 at 8:14

You can activate Photo Stream in iPhoto so you don't need to sync your device with your computer. In iPhote you then copy the pictures to a new album,
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HoshtsAug 16 '12 at 8:58

Keep in mind as well that you are able to backup the iPad to either your computer (via iTunes) or to iCloud, both of which will backup your photos, settings, messages, etc. You can also import your photos directly into iPhoto (assuming you have a Mac) as your iPad will show up as a camera when connected.
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Mr RabbitMar 22 '13 at 16:05